Analysis of the characteristics of trytophan antimetabolites on cell culture of L1210 murine leukemia cells revealed a unique feature of 4- fluorotryptophan not shared by its 5- and 6- fluoro- positional isomers. Only the 4- isomer was cytotoxic in culture at concentrations equal to that of its natural metabolite, tryptophan, a characteristic which should make it effective in vivo. Cytotoxicity was manifested by growth inhibition, cell swelling and lysis. In vivo studies employing continuous release pellets implanted SC indicated its limit for host toxicity. Reports that cytotoxity of interferon-tau is due to induction of indoleamine-2, 3- oxygenase which degrades cellular tryptophan and acts by causing a tryptophan deficiency has prompted initial studies on combination chemotherapy with the biological response modifier and antimetabolite.